Geographic Atrophy (GA) is an advanced form of dry age-related macular degeneration (AMD), a common eye condition affecting older adults. It involves the progressive and irreversible loss of retinal tissue, specifically the photoreceptors, retinal pigment epithelium (RPE), and underlying choriocapillaris. This deterioration can lead to a gradual loss of central vision over time. Optical Coherence Tomography (OCT) is a non-invasive imaging technology widely used by ophthalmologists to diagnose and monitor this condition.
Understanding Optical Coherence Tomography
Optical Coherence Tomography operates like a light-based ultrasound, creating detailed, cross-sectional images of the retina’s layers. The OCT machine projects a low-power, near-infrared light onto the eye, which then penetrates the retinal tissue and reflects back. By analyzing the time delay and intensity of these reflected light waves, the system constructs a precise, layered map of the retina’s internal structures. This technology allows eye care professionals to visualize and measure the thickness of retinal layers.
For the patient, undergoing an OCT scan is a straightforward experience. The procedure is non-invasive, meaning no contact is made with the eye and it is painless. Patients sit in front of the machine, resting their chin on a support, and focus on a small target light. The scan itself is quick, taking only a few minutes per eye, and does not require injections. Sometimes, dilating eye drops may be used to widen the pupil for a clearer view, which might cause temporary light sensitivity or blurry vision.
Identifying Geographic Atrophy on an OCT Scan
When examining an OCT scan for Geographic Atrophy, ophthalmologists look for several distinct features. A primary sign is the disappearance of the retinal pigment epithelium (RPE) and overlying photoreceptor cells. These layers, normally visible as distinct bands on an OCT image, become absent or severely thinned in areas of atrophy. The RPE, which supports the photoreceptors, is particularly affected, and its loss contributes to the characteristic appearance of GA.
A hallmark feature of GA on an OCT scan is “hypertransmission,” also known as the “waterfall” effect. This phenomenon occurs because the RPE layer, which normally absorbs and scatters light, is damaged or absent. With the RPE no longer impeding light, more of the OCT signal penetrates deeper into the underlying choroid. This increased light penetration makes the choroid appear hyper-reflective on the scan. Hypertransmission defects measuring 250 micrometers or more are significant, often predicting GA formation.
The thinning of the outer retina and the loss of the RPE also leads to enhanced choroidal visualization. In a healthy eye, the RPE partially obscures the choroid, a layer of blood vessels. In areas of GA, the absence of the RPE allows for a clearer view of choroidal blood vessels and other structures. This increased visibility, along with hypertransmission, provides evidence of geographic atrophy. The Classification of Atrophy Meetings (CAM) Group has established specific criteria to standardize the identification of these changes on OCT.
Measuring and Monitoring Disease Progression
After an initial diagnosis, OCT scans become an important tool for ophthalmologists to measure and track the progression of geographic atrophy over time. Specialized software can delineate the boundaries of atrophic lesions on OCT images, allowing calculation of their total area. This quantitative measurement provides a baseline against which future scans can be compared.
By performing serial OCT scans at regular intervals, eye care professionals can determine the rate at which atrophic lesions are enlarging. The mean annual enlargement rate of GA can vary. Comparing these sequential images allows calculation of the lesion’s growth rate, providing objective data on the disease’s trajectory. Tracking this progression is important for understanding how the disease affects vision and predicting future vision loss.
The Role of OCT in GA Treatment and Research
Optical Coherence Tomography plays an important role in guiding treatment decisions for geographic atrophy and advancing research into new therapies. The measurements provided by OCT scans determine a patient’s eligibility for newly approved treatments for GA. These treatments aim to slow the progression of lesion growth. OCT helps clinicians assess the size and characteristics of the atrophy, ensuring that patients meet the specific criteria for these therapies.
OCT is an important tool in clinical trials for GA. It serves as a key endpoint to measure the effectiveness of investigational drugs at slowing lesion expansion. Researchers use OCT to assess changes in atrophic area and retinal layer integrity, providing objective data on whether a new drug achieves its therapeutic goals. This structural information from OCT is often correlated with functional outcomes to understand the impact of new treatments.
Advanced OCT technologies, such as OCT Angiography (OCT-A), also contribute to GA research. OCT-A provides visualization of the blood vessels in and under the retina, including the choriocapillaris, without the need for dye injections. This allows researchers to study blood flow changes in the context of GA, revealing new insights into the disease’s mechanisms and identifying new biomarkers for progression. These insights help pave the way for targeted and effective treatments.